In fuel cells, e.g., polymer electrolyte fuel cells, one side of an electrolyte membrane is exposed to a fuel gas such as hydrogen, and another side thereof is exposed to an oxygen gas.
As a result, in polymer electrolyte fuel cells, water is produced through a chemical reaction taking place in the electrolyte membrane.
Thus, polymer electrolyte fuel cells are based on electrical extraction of reaction energies produced in this way.
Cell units in polymer electrolyte fuel cells each have membrane-electrode assemblies (hereinafter, referred to as MEAs), and pairs of separators each located on both sides of MEAs.
MEAs each include: hydrogen-ion-conductive polymer electrolyte membranes; pairs of electrode layers that each hold the polymer electrolyte membranes; catalyst layers; and gas-diffusion layers. Catalyst layers contain carbon powders carrying platinum-group catalysts as main ingredients and are formed on both sides of the polymer electrolyte membranes. The gas-diffusion layers are formed on the catalyst layers and combine power-collection actions, gas permeability, and water repellency.
It is required that the gas-diffusion layers have sufficient gas permeability and gas-diffusion properties, such that the gas-diffusion layers uniformly supply gases coming from the separators to the catalyst layers.
Moreover, it is also required that the gas-diffusion layers have excellent electric conductance so as to serve as electron-conducting pathways between the gas-diffusion layers and the catalyst layers.
Therefore, electrically-conductive porous substrates such as carbon-fiber unwoven fabrics, and carbon-fiber woven fabrics have been employed for the gas-diffusion layers.
Furthermore, it is required that the gas-diffusion layers have high water repellency, so as to remove excess water produced through battery reactions in the catalyst layers from the MEA systems, thereby preventing the produced water from blocking pores of the gas-diffusion layers.
Therefore, materials obtained by subjecting electrically-conductive porous substrates to water-repellent treatments with fluorine resins have been employed for the gas-diffusion layers.
Additionally, water-repellant layers that contain as main ingredients carbon powders, and water-repellant resins such as fluorine resins, are frequently provided at sides adjacent to the catalyst layers, which are made of electrically-conductive porous substrates.
Thus, since the electrically-conductive porous substrates for gas-diffusion layers are subjected to water-repellant treatments, blockage of pores of the gas-diffusion layers by the produced water is prevented.
Additionally, since the water-repellant layers have higher water repellency than the electrically-conductive porous substrates, it becomes possible to quickly discharge the excess water produced in the catalyst layers to the outside of the MEA systems.
JP-A-2003-197202 discloses gas-diffusion layers in which water-repellant layers are formed on electrically-conductive porous substrates.
In the gas-diffusion layers, water-repellant layers that includes carbon materials (e.g., carbon black, hollow carbon fibers) and fluorine resins are formed on surfaces of carbon-fiber based substrate ‘e.g., papers, woven fabrics, unwoven fabrics).
Furthermore, WO2010/050219, JP-A-2003-187809, and JP-A-2007-141783 each disclose gas-diffusion layers in which no carbon fibers are used as substrates.
The gas-diffusion layers disclosed in WO2010/050219 are formed of porous materials that include as main ingredients electrically-conductive particles, and polymer resins, and further includes smaller amounts of carbon fibers compared with the amounts of polymer resins.
The gas-diffusion layers disclosed in JP-A-2003-187809 are formed by mixing graphite, carbon black, unbaked PTFE and baked PTFE.
The gas-diffusion layers disclosed in JP-A-2007-141783 are formed of boron-modified carbon, carbon fibers, and fluorine resins.
Furthermore, the publication of Japanese patent No. 5,099,017 discloses gas-diffusion layers that are formed of porous materials. The porous materials are produced through foam molding of foamable slurries containing starting material powders of corrosion-resistant metals, foaming agents, and binders, and thus, have three-dimensional network structures in which hollows communicating with each other are present.
The publication of Japanese patent No. 5,476,694 discloses gas-diffusion layers that are made of water-repellant porous materials having recessed parts and projecting parts. The most frequently distributed diameters of hollows present in the projecting parts are larger than those of hollows present in the recessed parts in the porous materials.
JP-A-2005-267902 and JP-A-2011-243314 disclose gas-diffusion layers having pore diameters set to predetermined ranges.
In the gas-diffusion layers disclosed in JP-A-2005-267902, a diameter of pores having a high-frequency peak of pore volume is adjusted to a range from 10 μm to 30 μm.
In the gas-diffusion layers disclosed in JP-A-2011-243314, a mean pore diameter is adjusted to a range from 31 μm to 49 μm.